This invention relates generally to an apparatus and method for lifting objects and, more particularly, to an apparatus and method for lifting heavy objects with an extendable boom gantry.
Various systems and methods for lifting heavy objects have been developed over thousands of years. Traditional gantries are well known in the prior art for their usefulness for lifting heavy objects. However, a satisfactory system for vertically extending the gantry boom has eluded those skilled in the gantry art. In particular, a number of design concerns must be addressed in developing a satisfactory gantry.
One prior art design utilizing jacks that can be in the form of telescopic lifting rams is disclosed by German Patent No. 2439667. While this structure allows vertical extension, structural limitations of traditional hydraulic cylinders may allow this structure to lift a heavy load to a point where the structure can no longer support the load. This is at least partly attributable to the fact that traditional hydraulic cylinders are designed to be strong in tension and compression, but fail at relatively low bending stress. Accordingly, a relatively simple structure for vertically extending the boom on a gantry while providing sufficient strength to withstand bending stress is needed.
To achieve substantial vertical lift heights, multi-stage hydraulic cylinders can be used. However, multi-stage hydraulic cylinders are expensive and heavy and the expense and weight increases with the overall extended length of the cylinder. Accordingly, it is desirable to provide a method of extending a lifting structure which is capable of extension greater than the extended length of the hydraulic cylinder.
Still another design concern is that it is difficult to precisely coordinate the extension of two hydraulic cylinders. In addition, it is difficult to coordinate two sets of driving wheels. Such lack of coordination can cause structural failure when a rigid gantry structure is attached to said mechanisms. Accordingly, means for allowing slight misalignments while retaining the structural integrity of the gantry structure are critically needed.
Finally, hydraulic systems occasionally fail by losing pressure. In a gantry system, such failure can cause catastrophic results because the load may drop in conventional designs. Accordingly, a safety feature which is independent of the hydraulic system for its holding force while being actuated by the hydraulic system failure is needed.
It is therefore an object of the invention to provide a new and improved method for lifting heavy objects with an extendable boom.
It is another object of the invention to provide a method of operating a vertically extendable gantry capable of extension greater than its lift cylinder extension range.
In accordance with one aspect of the invention, this object is achieved by supporting a load on a first section of an extendible lift assembly, the first section being surrounded by a second section, a hydraulic cylinder being operatively coupled to the first section so as to be capable of lifting the first section, and the first and second sections being supported on and vertically extendible with respect to a base. Then, while the load is supported on the first section, the hydraulic cylinder is extended to extend the first section relative to the second section and the base, thereby lifting the load to a first height. Then, the first section is secured to the second section so as to at least substantially prevent downward movement of the first section relative to the second section. The cylinder is then retracted and operatively coupled to the second section. Then, while the load is supported on the first section, the hydraulic cylinder is extended to extend the first and second sections relative to the base, thereby lifting the load to a second height which is higher than the first height. This procedure permits a load to be lifted through a range which is greater than a maximum range of extension of the hydraulic cylinder.